nextpnr: Add eigen dependency.
nextpnr: Add eigen dependency.

Signed-off-by: Tim 'mithro' Ansell <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@mith.ro>

litex-hub/litex-conda-prog,litex-hub/litex-conda-compilers,timvideos/conda-misoc-lm32,timvideos/conda-hdmi2usb-packages,timvideos/conda-misoc-lm32,timvideos/conda-hdmi2usb-packages,litex-hub/litex-conda-eda